1. Field of the Invention
The present invention relates to quantum cavity light emitters and, more particularly, to a quantum cavity light emitter having a predetermined shape and dimensions such that spontaneous light emission is enhanced in some directions and suppressed in other directions.
2. Description of the Prior Art
Microcavities are optical cavities having dimensions on the order of an optical wavelength. In microcavities, the spontaneous emission of light can be strongly modified. The modification of spontaneous emission is best understood as follows. Spontaneous emission is really stimulated emission. In the absence of any optical medium, the weak, randomly-oriented "vacuum fluctuation" electromagnetic fields provide the stimulation, producing light emission equally in all directions at a rate determined by the strength of the fields. These vacuum fields are present virtually everywhere unless significant effort is taken to suppress them. When the vacuum fields are incident on an optical structure, such as an optical cavity, they are modified in the same manner as any electromagnetic wave incident on the structure. When light is incident upon a planar Fabry-Perot optical cavity at the cavity resonance frequency, the optical intensity inside the cavity is much higher than the incident intensity. At different incident angles, the resonant frequencies are different. Light incident from the side of the cavity sees no cavity at all, and the light intensity is not strongly modified. Therefore in a planar Fabry-Perot cavity, spontaneous emission is strongly enhanced along the cavity axis at the normal-incidence resonant frequency. Spontaneous emission is also enhanced at a range of angles deviating from the cavity axis at higher optical frequencies (shorter wavelengths). Of critical importance to understanding modification of spontaneous emission in a cavity, is the fact that the on axis spontaneous emission is suppressed at off-resonant frequencies. At angles perpendicular to the cavity axis, the spontaneous emission is not strongly modified. However the emission occurs for all frequencies at which the material can emit. Thus the total emission perpendicular to the cavity axis, the "lateral modes," can greatly exceed the emission normal to the cavity axis, the "axial modes." In most devices, the axial modes represent the desired or useful emission, while light emitted into the lateral modes is not used and therefore considered wasted, however, in conventional light emitting devices utilizing optical cavities, most of the spontaneous emission is emitted into the lateral modes and therefore wasted.
A general discussion of microcavities is found in the article by Yamamoto and Slusher, entitled "Optical Processes in Microcavities." The article provides an introduction to cavity quantum electrodynamics, which describes the spontaneous emission processes in microcavities, and describes the three major classes of microcavities: micro-Fabry-Perot cavities, "whisper-gallery" cavities, and photonic bandgaps. Micro-Fabry-Perot cavities, typically called vertical cavities due to their similarity with vertical-cavity surface-emitting lasers (VCSELs), produce counter-propagating standing waves along the cavity axis. Vertical cavities may be planar with negligible lateral structure, or they may additionally have lateral reflectance. Vertical cavities with lateral reflectance are the structures which are improved in the present invention. Whisper-gallery cavities produce traveling waves which circulate around the typically circular cavity. Photonic bandgaps provide reflectance in essentially all directions, and their fabrication generally requires complex three-dimensional fabrication processes.
The article by Zhang et al., "Microcavity Vacuum-Field Configuration and Spontaneous Emission Power," describes theoretical and experimental analysis of spontaneous emission in vertical cavities. Although the vertical cavities have a lateral surface which provides some lateral reflectance, the lateral dimensions are so large, 15 to 20 micrometers, compared to the wavelength of the light in the material, about 0.25 micrometer, that these are essentially planar vertical cavities. The article theoretically predicts that a maximum of 11% of the light emitted goes into the axial modes. The experiments report a maximum of 8.5% emission into the axial modes.
The article by Jewell et al., entitled "Transverse Modes, Waveguide Dispersion, and 30 ps Recovery in Submicron GaAs/AlAs Microresonators," describes some effects seen in vertical microcavities with laterally defined diameters ranging from less than 0.5 micrometers to 1.5 micrometers. Transverse axial modes were reported, which were separated in wavelength by 6 nanometers for a 1.4 .mu.m diameter device, and by 7.4 nm for a 1.2 .mu.m diameter device. The waveguiding optical confinement also shifted the fundamental axial mode resonance to shorter wavelengths, in agreement with a numerical simulation. Waveguides with smaller diameters have larger shifts of their axial modes.
The article by Baba et al., entitled "Spontaneous Emission Factor of a Microcavity DBR Surface-Emitting Laser." theoretically describes the spontaneous emission from vertical cavities with lateral optical confinement. The lateral optical confinement is provided by etching the structures, thereby forming sidewalls. The sidewalls will necessarily reflect light incident upon them. In this article, effects due to the optical waveguiding confinement are described; however, effects arising from the lateral, or horizontal, cavities formed by the reflecting sidewalls are not considered. The article describes the fraction of light which is emitted into the fundamental axial mode, predicting it to increase monotonically with decreasing lateral dimension. To achieve a high fraction of light into the fundamental axial mode, the device must be of submicron lateral dimensions and operate at low temperature.
The article by Fan et al., entitled "Design of Three-Dimensional Photonic Crystals at Submicron Lengthscales," describes a three-dimensional photonic bandgap structure which is designed for simple fabrication. While previously described photonic bandgaps involved etching of deep holes at multiple angles, the photonic bandgap described in the article required only a single vertical etch. However, even before the deep hole etching process is performed, the material to be etched first had to be prepared through a multiple etch-deposition process, because the device relies upon a significant amount of lateral structure. This structure is therefore not considered manufacturable, nor is it a vertical microcavity.
The article by Wendt et al., entitled "Nanofabrication of Photonic Lattice Structures in GaAs/AlGaAs, describes a two-dimensional photonic bandgap structure oriented in the plane of a semiconductor wafer. The photonic bandgap was fabricated by etching circular holes in a hexagonal pattern. The three photonic bandgap designs reported all involved extremely small hole diameters, or extremely small separations between holes, or both. The largest holes were less than half the vacuum optical wavelength, or 370 nm in diameter, and the largest separations were less than 1/8 the vacuum optical wavelength, or 100 nm. Fabrication was, therefore, extremely difficult and was successful in only one of the three designed structures, which had 245 nm diameter holes and 50 nm separations. For each of the three designs, the lattice constant was about half the optical wavelength in the composite material.